Hey, thanks for commenting. I apologize for not directly answering your question. I just wanted to stress that you won't be able to come up with a response for every situation. BUT... you're right... what do you do when someone asks a question that makes it feel like time stops and you don't know what to say.
The one thing you should never say is that you don't know the answer. By that I mean saying exactly the words "I don't know". My rule of thumb is this - When I'm asked a question that I'm uncertain how to respond to, I switch gears and look at the situation from the reverse. For instance, when I interviewed for my first job in higher education I was asked how I would handle a student who was upset about not getting a class they wanted. Now, I had never worked in higher education so it wasn't a situation I'd ever handled, but I imagined how I would my advisor to respond if I was the upset student. In fact, you could even reframe your answer that way which is what I did. In effect, what I said was "Well, if I were the student I would want my advisor to do "X" which I went to explain. Then I followed-up that up by saying I would attempt to emulate what I would someone to do if I were in that position.
Of course there may be times when a question comes at you that makes this impossible. Let's say you're applying for an operations internship at a logistics company and the interviewer asks you how you would handle a situation where the dispatch team is on strike and you've got 40 trucks awaiting further instructions for delivery, and you're asked to fill in as a dispatcher. Enough to cause panic, right? Well, what you do is turn this into a game of Jeopardy where you answer the question with another question by asking questions like "Is the supervisor of the dispatch team on strike?" If so, you could ask who next in line would be and on it on it goes.
To me, the key is not knowing the "exact" technical answer for every question. That's unrealistic and it's very unlikely that's what your interviewer is looking for. The thing to remember is that businesses do not operate linearly - there is always some kind of contingency to deal with whether it's losing a big client, worker attrition, and so on. By asking questions to help you develop your response, rather than just nervously rambling off an answer, shows that you have the ability (and the willingness) to think things through systematically AND that you are willing to seek out help when you need it most. That's the kind of employee I'd want on my team.